This invention relates to series-parallel condensing systems used in thermal power stations that employ both air-cooling and evaporative cooling, and more specifically, to a system that is more efficient, less costly and easier to operate than current state of the art parallel condensing systems.
Thermal power stations throughout their history have utilized the Rankine steam cycle to generate electric power. This involves massive rejection of waste heat to the environment. In early designs this was accomplished by first condensing the steam exiting the turbine in a surface condenser and then transporting the heat by means of a circulating water systems to large bodies of water, rivers or cooling towers. Such systems were relatively low in cost and also allowed for efficient plant operation in the warmer parts of the year. Therefore the use of wet evaporative based cooling systems was standard practice for more than half a century until the feasibility of siting ever-larger power plants became problematical with respect to water availability or environmental impact. This led to the gradual introduction of air-cooled condensers, which required no water and had minimal environmental impact, but unfortunately increased the cost of the power plants and also resulted in a loss of electric generation, particularly during the warm periods of the year.
To mitigate the disadvantages associated with all dry air-cooled condensing and the water and environmental problems associated with wet evaporative cooling, parallel condensing systems were introduced in the early 1990's. Such systems employ an air-cooled condenser and a surface condenser that are connected via parallel steam paths to the plant's turbine's exhaust. The surface condenser in turn is connected in a conventional manner via a circulating water system to a cooling tower, body of water, or other water based heat sink.
Such parallel condensing systems could readily be designed to vary the fraction of heat rejected by the wet and dry sections of the system depending on water availability or environmental constraints. Furthermore, since water availability was generally based on annual limitations, the water consumption profile could be shaped to maximize use of the wet evaporative section in the warm part of the year to make up for the loss in performance in the air-cooled condenser during these conditions. In typical parallel condensing system applications the cost of the air-cooled condenser was cut dramatically, annual water consumption was reduced by two thirds or more and plant output during warm weather was nearly the same as for all-wet evaporative cooling. In addition, water related environmental impacts were highly reduced resulting in greater plant siting flexibility and faster plant permitting cycles.
The air-cooled condensers employed in parallel condensing systems of the type described above utilized a two stage series condensing process commonly referred to as a K-D condensing process. A brief description of this process follows.
The two-stage K-D condensing process was devised in order to eliminate so called “dead zones” in air-cooled condensers in which no condensation takes place. In the K-D process steam first enters the K section heat exchangers from above and in which steam and forming condensate flow in the same direction. By limiting the length of the K tubes and by properly modulating airflow, condensation is not allowed to complete in this section and some steam exits all fin tube rows at the bottom under all operating conditions along with draining condensate.
Steam leaving the K section is collected in a header that transports the steam into the second stage, commonly referred to as a dephlegmator or D stage, where steam enters the heat exchangers from below. Steam and forming condensate flow in counterflow direction to each other in this section. The size of the D section can vary between as little as 1/10 to ⅓ or more of the overall deployed condenser heat transfer surface depending on climatic and plant loading conditions. Condensation finally completes near the very top of the D section with the remaining upper interior tube volume being filled with non-condensibles, principally air. Non-condensibles are continuously removed by ejection equipment. During sub-freezing atmospheric conditions the remaining moisture contained in the non-condensibles freezes on the cold tube walls in the form of a soft rime ice and slowly closes the tube passages. Without any further active measures being taken this would result in a cessation of air removal from the dephlegmator tubes, followed by air filling the entire D section, followed by intrusion of air into the K section. The condenser would now be subject to serious freeze damage and performance degradation. In order to prevent the above noted freezing problems from occurring it is necessary engage in a continuous active dephlegmator warming program. This consists of periodically reducing the speed of the fans serving D sections. This results in steam flooding of the upper end of the D tubes, which melts the rime ice. This procedure, although generally solving the problem of freeze damage to the condenser, results in significant control system complications and also very demanding operator attention during cold weather periods. In addition the frequent airflow modulation required in the D sections causes fluctuations in turbine backpressure that affect plant output and reliability.
The condensate formed in both the K and D sections initially drains into the common bottom header connecting these sections. This condensate is somewhat sub-cooled due to contact with cold tube surfaces. The condensate is collected in the header and then routed to the condensate tank in a system of drainpipes that are normally heat traced and insulated to prevent condensate freeze-up during cold weather. Even though the drainpipes are heat-traced and insulated, additional sub-cooling of condensate still occurs in the drain lines. Sub-cooling of condensate is deleterious because it decreases thermodynamic efficiency and, more importantly, increases the dissolved oxygen content of the condensate. Dissolved oxygen in the condensate creates serious corrosion problems in the overall steam cycle. Therefore separate condensate deaerators are frequently required and incorporated in the drain systems of K-D condensing systems to control the amount of sub-cooling, adding complexity and cost.
Although the K-D system satisfies the crucial requirement of minimizing unwanted “dead zones” in the condenser and providing reliable operation in extreme cold weather, inherently high internal steam side pressure drops degrade its performance. These result from the fact that the steam must pass in series through two stages of fin tubes plus a steam transfer header, producing considerable friction losses plus additional turning and acceleration losses leaving and entering the two sets of fin tubes. These parasitic pressure losses produce a corresponding drop in the saturation temperature of the steam, which reduces the temperature difference potential between steam and cooling air, and thus the efficiency of the air-cooled condensing system.
In addition to the complications already noted several additional features must typically be incorporated in K-D systems for proper and safe operation. These include a condensate collection tank to collect the condensate draining from the transfer headers, a pressure equalizing line between turbine exit and the condensate tank and draining facilities to continuously remove condensate from of the main steam duct to the condensate tank.